Aggregation-Induced Intersystem Crossing: Rational Design for Phosphorescence Manipulation

Phosphorescence of organic molecules has drawn extensive attention due to its potential applications in energy and life science. However, typically intersystem crossing (ISC) in organic molecules is slow due to the small spin-orbit couplings (SOC) and large energy gaps (ΔES-T) between different multiplicities. Molecular aggregation offers a practical strategy to manipulate phosphorescent characteristics. In this work, the impact of aggregation on the luminescence properties of π-conjugated benzophenone luminophore 1-dibenzo[b,d]thiophen-2-yl(phenyl)methanone (BDBT) are investigated theoretically using density functional theory (DFT) and time-dependent DFT. Molecular aggregation results in substantial energy splitting and variation of SOC, eventually changing the ISC rate. This is known as the "aggregation-induced intersystem crossing" (AI-ISC) mechanism. Different types of electron donating and withdrawing functional groups are further introduced into BDBT molecular system to tailor the phosphorescent efficiency. We find that functional groups can influence the SOC and energy gaps and further manipulate the phosphorescence efficiency. Molecular systems with donating functional groups have faster ISC rates, and dimers exhibit the best electronic luminescence due to the relatively large SOC and small ΔES-T. The AI-ISC mechanism accompanied by group functionalization provides a practical platform for phosphorescence enhancement.